| Oak Grove Folk Music Festival | Staunton | VA | United States | August | 2nd Weekend | 3 Days | Bluegrass | Music |
| Oceans Calling Festival | Ocean City | MD | United States | September | 4th Weekend | 3 Days | Indie, Pop, Rock | Music |
| Off Beat Music Festival | Reno | NV | United States | October | 1st Weekend | 3 Days | Indie | Music |
| Ogden Music Festival | Ogden | UT | United States | June | 1st Weekend | 3 Days | Bluegrass | Music |
| Ohana Festival | Dana Point | CA | United States | September | 4th Weekend | 3 Days | Alternative, Country, Folk, Indie, Rock | Music |
| OKC Roots Music Festival | Oklahoma City | OK | United States | October | 1st Weekend | 3 Days | Americana, roots, rock, blues, and folk, featuring singer-songwriter artists. | Music |
| Oklahoma’s International Bluegrass Festival | Guthrie | OK | United States | October | 2nd Weekend | 3 Days | Bluegrass | Music |
| Ol’ Front Porch Music Festival | Oriental | NC | United States | October | 1st Weekend | 2 Days | Bluegrass | Music |
| Old Fiddler’s Convention | Galax | VA | United States | August | 2nd Weekend | 6 Days | Bluegrass | Music |
| Old Settler’s Music Festival | Tilmon | TX | United States | April | 3rd Weekend | 4 Days | Bluegrass | Music |
| Old Settler’s Music Festival | Dale | TX | United States | May | 2nd Weekend | 4 Days | Bluegrass | Music |
| Old Time Music Ozark Heritage Festival | West Plains | MO | United States | June | 2nd Weekend | 2 Days | Bluegrass | Music |
| Old Town Music and Arts Festival | Williamsburg | OH | United States | August | 2nd Weekend | 2 Days | Bluegrass | Music |
| Olde Dominion Bluegrass Festival | Chatham | VA | United States | November | 3rd Weekend | 1 Day | Bluegrass | Music |
| Oldtone Roots Music Festival | Hillsdale | NY | United States | September | 1st Weekend | 4 Days | Bluegrass | Music |
| Opelika Songwriters Festival | Opelika | AL | United States | May | 3rd Weekend | 4 Days | Singer-Songwriter, Americana, Folk and Country | Music |
| Orange Army Bluegrass Festival | Flemingsburg | KY | United States | August | 1st Weekend | 3 Days | Bluegrass | Music |
| Orchards Park-Bentonville’s 4th of July Fireworks Celebration | Bentonville | AR | United States | July | 4th of July Weekend | 3 Days | A variety of genres are often represented, including patriotic music, country, rock, and pop. | Community Festival |
| Oregon Country Fair | Veneta | OR | United States | July | 2nd Weekend | 3 Days | Bluegrass, Folk | Music |
| Oregon Jamboree | Sweet Home | OR | United States | August | 1st Weekend | 3 Days | Country | Music |
| Osceola Music Festival | Osceola | IN | United States | August | 2nd Weekend | 3 Days | Bluegrass | Music |
| Ossippee Valley Music Festival | South Hiram | ME | United States | July | 4th Weekend | 4 Days | Bluegrass, but also features roots, Americana, folk, blues, and world music. | Music |
| Outlandia Festival | Bellevue | NE | United States | August | 2nd Weekend | 2 Days | Indie, Rock | Music |
| Outlaw Country Cruise | Miami | FL | United States | February | 3rd Weekend | 7 Days | Country, Rock | Music |
| Outside Lands | San Francisco | CA | United States | August | 2nd Weekend | 3 Days | Alternatic, Electronic, Hip Hop, Inide, Rock | Music |
| Oyster Ridge Music Festival | Kemmerer | WY | United States | July | 3rd Weekend | 2 Days | Bluegrass | Music |
| Ozark Folk Festival | Mountain View | AR | United States | September | 1st Weekend | 3 Days | Folk music, with elements of Americana, singer-songwriter, and roots music | Music |
